For the reproduction of information recorded on a magneto-optical disk, the Kerr effect of reflected light, which is a magneto-optical effect, is conventionally utilized.
FIG. 1 is a diagram explaining the reproduction principle of such a conventional magneto-optical disk.
In FIG. 1, reference numeral 1 denotes a semiconductor laser, numerals 2, 4, and 5 denote lenses, numeral 3 denotes a polarizer, numeral 6 denotes an analyzer, numeral 7 denotes a photodiode, numeral 8 denotes incident light, numeral 9 denotes reflected light, and numeral 10 denotes a perpendicular magnetic recording film.
As shown in FIG. 1, in the reproduction principle of a magneto-optical disk, the polarization plane of the reflected light 9 rotates with respect to that of the incident light 8 under the Kerr effect. The rotation angle of the polarization plane of the reflected light 9 is read and thereby storage is reproduced. Here, the rotation angle exhibits a maximum value when the direction of magnetization and the traveling direction of light are parallel to each other. For a recording film, therefore, a material having a magnetization perpendicular to the surface of a memory medium is desirable. The conditions of having a magnetization perpendicular to the surface in this way, offers the advantages of increasing the surface density and allowing the achievement of high density recording. Therefore, the perpendicular magnetic recording system will become mainstream henceforward.
The memory capacity of a magneto-optical disk depends on the spot size of a semiconductor laser used for reproduction. The typical reproduction wavelength of the semiconductor laser is in the range of 0.78 to 0.65 μm. In terms of reading accuracy, the size of magnetization is limited to the order of reading wavelength. This results in the limitation of recording capacity, thus constituting the largest problem to be solved henceforth.
On the other hand, an invention such as a magnetically induced super resolution (MSR) system is disclosed. Use of this system is making it possible to read out even a magnetization size of about a half of the typical reproduction wavelength of a semiconductor laser. According to K. Shono [J. Magn. Soc. Jpn. 19, Supple, S1 (1999) 177], a recording mark of 0.3 μm was reproduced at a wavelength of a red laser, and a recording capacity of 1.3 GB was implemented with a MO disk of 3.5 inch. However, this is also a reading size of about a half of the wavelength at most, and therefore, it is difficult to reproduce a minute magnetization size of not more than 0.1 μm (1000 Å). Hence, this system has also its limit in recording capacity as a natural result, thus constituting a serious problem to be solved, as well.
According to conventional methods in which the magneto-optical effect is utilized for the reproduction of information, semiconductor laser light is directly made incident onto a magneto-optical disk on which records are written. When the temperature increased by this incident light has reached a temperature of not less than the spin alignment temperature (Curie temperature Tc) of the magnetic material of the magneto-optical disk, the storage is unfavorably erased. Accordingly, a problem occurs in that the intensity of the incident light for reading must be limited so that this transition temperature Tc or more is not reached. This results in imposing restrictions on the S/N ratio improvement of reproduction signals, thereby causing an excessively high load on a reproduction signal processing system.
While the foregoing concerns problems associated with the reproduction of recording data on the magneto-optical disk, the reproduction device using a magnetoresistive mechanism of a hard disk device (HDD) has also similar technical problems. With the trend moving toward increasingly minuter configurations of magnetic materials for recording, it is necessary for the reproduction to read magnetism in ultrafine areas with high sensitivity.
As a next generation technology for reading HDD data, a tunneling magneto resistive (TMR) head (Fujikata et al., The 8th Joint MMM-Intermag Conference Abstracts, p. 492, January 2001), and as a technology of the generation after the next generation, an extraordinary magneto resistive (EMR) head are being developed in a fierce competition among manufacturers.
Even in this EMR, which is referred to as a technology of the generation after the next generation, at the stage of prototyping stage, the diameter of a reading element is several millimeters (Solin et al., Science, vol. 289, pp. 1530–1532, September 2000), and the reading of a magnetization size of not more than 0.1 μm (1000 Å) will be a challenge for the future. The commercialization of EMR, therefore, will be a long way into the future.